So-called screws, which are comprised by forming a thread along spiral path, namely a helix, depicted on the outer circumferential surface of rod by moving the center of a circular path along the central axis of the rod, have proliferated extensively as fundamental components used in industry. When broadly categorized according to the direction in which their threads are wound, screws are classified into right-handed screws, in which the thread extends clockwise in the positive axial direction, and left-handed screws, in which the thread extends counter-clockwise in the positive axial direction.
The main applications of these screws include fastening applications for tightening mechanical parts and the like, transfer applications and movement applications for transmitting motive power or movement, as well as other applications such as measurement and adjustment applications for measuring dimensions or adjusting position by utilizing the correlation between screw rotation and distance moved in the axial direction.
There have conventionally been various types of screws respectively designed to be suitable for each application. For example, triangular threaded screws, in which the shape of the thread is triangular, are mainly used for fastening, measurement and adjustment. An example of a screw typically used for fastening mechanical parts is a metric coarse threaded screw, while fine threaded screws having a narrow pitch are used for fastening precision mechanical parts in particular. In addition, pipe screws, in which threads are formed in a pipe material, are used to connect pipes materials and the like. Rectangular threaded and trapezoidal threaded screws, in which the threads are formed into a rectangular shape or trapezoidal shape, are used for transmission of motive power or movement such as in presses or jacks. Round threaded screws, in which the threads are formed to have a rounded shape, are used for light bulb bases and locations requiring ease of installation and removal such as connectors susceptible to infiltration of foreign objects. In addition, an example of a special type of screw is a ball screw, in which a large number of holes are formed along helical spiral grooves, and which is designed so as to reduce frictional resistance during thread meshing by incorporating steel balls in the holes while allowing to rotate freely.
Fastening screws are composed of a combination of an external thread, comprised by forming a helical spiral groove in the outer circumferential surface of a rod, and an internal thread, comprised by forming a helical spiral groove in the cylindrical inner circumferential surface of a cylindrical material, and at the time of use, the external thread is inserted into a threaded hole formed in the material to be fastened, after which the internal thread is screwed onto this external thread, and the material to be fastened into which the external thread has been previously inserted is tightened with the external thread to achieve fastening. Examples of such fastening screws that have been designed to be able to be used easily include combinations of so-called bolts, comprised by forming a head portion in the form of a hexagonal column on one end of an external thread and so-called nuts, in which the outer circumferential surface that screws onto the bolt is formed in the shape of a hexagonal column.
Right-handed screws have been nearly exclusively used in conventional screw applications including bolts and nuts used for fastening, while left-handed screws have been limited to special applications such as turnbuckles in which internal threads are formed rotating in different directions along the same axis on both ends thereof.
In addition, there are also applications for fasteners comprising the combination of bolts and nuts that require that the nut screwed onto the bolt not become loose. A fastener that has been designed to accommodate such applications is referred to as an anti-loosening nut. This type of fastener is composed so that it is difficult for a nut screwed onto a bolt to become loose by making suitable design contrivances to the nut.
Conventional anti-loosening nuts can be broadly classified into so-called single-nut types, which are composed so that locking effects are demonstrated with a single nut, and double-nut types, which are composed so that locking effects are demonstrated by a combination of two nuts.
As disclosed in Patent Document 1, a typical conventional single-nut type anti-loosening nut is composed by incorporating a mechanism that fulfills a role similar to that of a washer in the nut, and when screwed onto a bolt, a washer-like plate piece attached to one end of the nut inhibits loosening of the nut by demonstrating an urging force between the bolt thread and the nut.
As disclosed in Patent Document 2, a typical conventional double-nut type anti-loosening nut is composed of a first nut that has a reduced-diameter end composed to as to be able to be reduced in diameter, and a second nut that reduces the diameter of the reduced-diameter end by screwing onto the outer circumference of the reduced-diameter end of the first nut, and is composed so as to inhibit loosening of the nut by the second nut being screwed onto the first nut, which is first screwed onto the bolt towards the reduced-diameter end facing backward in the direction of travel, the reduced-diameter end of the first nut being reduced in diameter, and the inner circumferential surface thereof being firmly pressed by the thread of the bolt.
Patent Document 1: Japanese Patent No. 3946752
Patent Document 2: Japanese Utility Model Registration No. 3018706
Such conventional externally-threaded screws consisted only of right-handed screws, in which helical threads are formed in the clockwise direction, or left-handed screws, in which helical threads are formed in the counter-clockwise direction, and there has yet to exist a duplex threaded body, which is both a right-handed screw and a left-handed screw, in which a clockwise helical spiral groove and counter-clockwise helical spiral groove are formed in the outer circumferential surface of a single rod-like member. In particular, there has been none whatsoever of a duplex threaded body having a region in which a right-handed portion, which is a region provided with a right-handed thread, and a region in which a left-handed portion, which is a region provided with a left-handed thread, overlap.
In addition, in the case of conventional single-nut type anti-loosening nuts, although a certain degree of locking effects can be obtained with a single nut, since urging force is simply generated with a washer-like plate piece to tighten meshing of the bolt and nut in a single spiral groove on the outer circumferential surface of the bolt, loosening of the nut is not substantially prevented, while in the case of a double-nut type, which is ultimately similar to the case of a single-nut type, since a reduced-diameter end of a first nut is reduced in diameter by a second nut, and the inner circumferential surface provided with irregularities on the reduced-diameter end is simply pressed to tighten meshing between the nut and bolt in a single spiral groove of the outer circumferential surface of the bolt causing loosening to be inhibited by frictional force between the outer circumferential surface of the bolt and the inner circumferential surface of the nut, both of these cases had the problem of the nut ending up loosening gradually due to vibrations and the like.